Three-Dimensional Chip Stack and Method of Forming the Same

ABSTRACT

A three-dimensional chip stack includes a first chip bonded to a second chip to form an electrical interconnection therebetween. The bonded interconnection includes a first conductive pillar overlying a first substrate of the first chip, a second conductive pillar overlying a second substrate of the second chip, and a joint structure between the first conductive pillar and the second conductive pillar. The joint structure includes a first IMC region adjacent to the first conductive pillar, a second IMC region adjacent to the second conductive pillar, and a metallization layer between the first IMC region and the second IMC region.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/016,966 filed on Sep. 3, 2013, entitled “Three-Dimensional Chip Stackand Method of Forming the Same,” which application is herebyincorporated herein by reference.

BACKGROUND

In an attempt to further increase circuit density, three-dimensionalintegrated circuits (3DICs) have been investigated. In a typicalformation process of a 3DIC, two chips are bonded together andelectrical connections are formed between each chip and contact pads ona substrate. For example, bonding two chips may be accomplished byattaching one chip on top of the other. The stacked chips are thenbonded to a carrier substrate and wire bonds electrically couple contactpads on each chip to contact pads on the carrier substrate. However,this requires a carrier substrate larger than the chips for the wirebonding. More recent attempts have focused on flip-chip interconnectionsand the use of conductive balls/bumps to form a connection between thechip and the underlying substrate, thereby allowing high-wiring densityin a relatively small package. Traditional chip stacking using solderjoints involves solder, flux and underfill. All these processes createdissues and limitations on pitch, joint height, and flux residue.

DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are cross-sectional views at various stages of manufacturing afirst bump structure on a first chip according to at least oneembodiment;

FIGS. 5-6 are cross-sectional views at various stages of manufacturing asecond bump structure on a second chip according to at least oneembodiment;

FIGS. 7-10 are cross-sectional views at various stages of bonding thefirst chip and the second chip by attaching the first bump structure tothe second bump structure according to various embodiments;

FIGS. 11-13 are cross-sectional views at various stages of bonding thefirst chip and the second chip by attaching the first bump structurewith a hemisphere-shaped solder layer to the second bump structureaccording to at least one embodiment.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. The presentdisclosure may, however, be embodied in many different forms and shouldnot be construed as being limited to the embodiments set forth herein;rather, these embodiments are provided so that this description will bethorough and complete, and will fully convey the present disclosure tothose of ordinary skill in the art. It will be apparent, however, thatone or more embodiments may be practiced without these specific details.

In the drawings, the thickness and width of layers and regions areexaggerated for clarity. Like reference numerals in the drawings denotelike elements. The elements and regions illustrated in the figures areschematic in nature, and thus relative sizes or intervals illustrated inthe figures are not intended to limit the scope of the presentdisclosure.

FIGS. 1-4 are cross-sectional views at various stages of manufacturing afirst bump structure on a first chip according to at least oneembodiment.

Referring to FIG. 1, a first chip 100 includes a first semiconductorsubstrate 10A employed in a semiconductor integrated circuitfabrication, and integrated circuits may be formed therein and/orthereupon. The first semiconductor substrate 10A is defined to mean anyconstruction comprising semiconductor materials including, but notlimited to, bulk silicon, a semiconductor wafer, a silicon-on-insulator(SOI) substrate, or a silicon germanium substrate. Other semiconductormaterials including group III, group IV, and group V elements may alsobe used. The first semiconductor substrate 10A may further comprise aplurality of isolation features (not shown), such as shallow trenchisolation (STI) features or local oxidation of silicon (LOCOS) features.Examples of the various microelectronic elements that may be formed inthe first semiconductor substrate 10 include transistors (e.g., metaloxide semiconductor field effect transistors (MOSFET), complementarymetal oxide semiconductor (CMOS) transistors, bipolar junctiontransistors (BJT), high voltage transistors, high frequency transistors,p-channel and/or n-channel field effect transistors (PFETs/NFETs),etc.); resistors; diodes; capacitors; inductors; fuses; and othersuitable elements. Various processes are performed to form the variousmicroelectronic elements including deposition, etching, implantation,photolithography, annealing, and other suitable processes. Themicroelectronic elements are interconnected to form the integratedcircuit device, such as a logic device, memory device (e.g., staticrandom access memory or SRAM), radio frequency (RF) device, input/output(I/O) device, system-on-chip (SoC) device, combinations thereof, andother suitable types of devices. The first semiconductor substrate 10Afurther includes inter-layer dielectric layers (not shown) and metallines (not shown) overlying the microelectronic elements. Theinter-layer dielectric layers and the metallization structure includelow-k dielectric materials, un-doped silicate glass (USG), siliconnitride, silicon oxynitride, or other commonly used materials. Thedielectric constants (k value) of the low-k dielectric materials may beless than about 3.9, or less than about 2.8. Metal lines in thedielectric layers may be formed of copper or copper alloys.

FIG. 1 also shows a first under-bump-metallurgy (UBM) layer 12A formedon the first semiconductor substrate 10A. In some embodiments, the firstUBM layer 12A includes a stack of a first layer and second layer. Thefirst layer, also referred to as a diffusion barrier layer or a gluelayer is formed of titanium, tantalum, titanium nitride, tantalumnitride, or the like by physical vapor deposition (PVD) or sputtering.The first layer is deposited to a thickness ranging from about 500 to2000 Angstroms and, in some embodiments for example, to a thickness ofabout 1000 Angstroms. The second layer is a copper seed layer formed byphysical vapor deposition (PVD) or sputtering. The second layer may beformed of copper alloys that include silver, chromium, nickel, tin,gold, or combinations thereof. The second layer is deposited to athickness ranging from about 500 to 10000 Angstroms and, in someembodiments for example, to a thickness of about 5000 Angstroms. In atleast one embodiment, the first UBM layer 12A includes a Ti layer and aCu layer.

For pillar bump formation, a mask layer 14 is provided on the first UBMlayer 12A and patterned with an opening 15 for example, by exposure,development and/or etching, so that a portion of the first UBM layer 12Ais exposed. In an embodiment, the mask layer 14 is a dry film, and maybe formed of an organic material. In alternative embodiments, the masklayer 14 is formed of a photo resist. The thickness of the mask layer 14may be greater than about 5 μm, or even between about 10 μm and about120 μm.

As shown in FIG. 2, the opening 15 is then partially filled with aconductive material layer, which is referred to as a first conductivepillar 16A hereinafter. The first conductive pillar 16A is formed of aconductive material with solder wettability. In an embodiment, a firstconductive pillar 16A is a copper (Cu) layer which is intended tosubstantially include a layer comprising pure elemental copper, coppercontaining unavoidable impurities, and/or copper alloys containing minoramounts of elements such as tantalum, indium, tin, zinc, manganese,chromium, titanium, germanium, strontium, platinum, magnesium, aluminumor zirconium. The formation methods may include sputtering, printing,electro plating, electroless plating, and/or commonly used chemicalvapor deposition (CVD) methods. For example, electro-chemical plating(ECP) is carried out to form the Cu layer. In a representativeembodiment, the first conductive pillar 16A has a thickness less thanabout 20 μm, although the thickness may be greater or smaller. Inanother exemplary embodiment, the thickness of the first conductivepillar 16A is greater than 20 μm. In another exemplary embodiment, thethickness of the first conductive pillar 16A is greater than 40 μm.

The first conductive pillar 16A is a Cu pillar in accordance with anembodiment. In some embodiments, a metal capping layer is formed on thetop of the copper pillar to prevent oxidation and diffusion of copperto/from a bonding layer. FIG. 2A shows an alternative embodiment of thefirst conductive pillar 16A including a metal capping layer 17 b on thetop surface of a Cu pillar 17 a within the opening 15. The metal cappinglayer 17 b may be formed by electroplating or electroless depositionmethods. The metal to be deposited is not particularly limited. Themetal may be nickel (Ni), copper (Cu), silver (Ag), gold (Au), palladium(Pd), platinum (Pt), tin (Sn), zinc (Zn), Ruthenium (Ru), a noble metal,or a combination of these. In an embodiment, the metal capping layer 17b is a nickel (Ni) layer. In some embodiments, the metal capping layer17 b is a nickel alloy layer, for example nickel-palladium-gold(NiPdAu), nickel-gold (NiAu), nickel-palladium (NiPd) or other similaralloys.

Next, as shown in FIG. 3, a first solder layer 18A is formed on the topsurface of the first conductive pillar 16A. The following discussionreferences an embodiment such as shown in FIG. 2, but applies equally toan embodiment such as illustrated in FIG. 2A. The first solder layer 18Ais made of Sn, SnAg, Sn-Pb, SnAgCu, SnAgZn, SnZn, SnBi-In, Sn-In, Sn-Au,SnPb, SnCu, SnZnIn, or SnAgSb, etc. In one embodiment, the first solderlayer 18A is a lead-free solder material layer. The first solder layer18A has a thickness less than about 10 μm. In another embodiment, thethickness of first solder layer 18A is less than or equal to about 7 μm.

Next, as shown in FIG. 4, the mask layer 14 is removed, and then theuncovered portion of the first UBM layer 12A is etched. In the case ofthe mask layer 14 comprising a dry film, it may be removed using analkaline solution. Thereafter, the exposed portion of the first UBMlayer 12A is etched back using the resulting structure (including layers16A and 18A) as a mask by a wet and/or dry etching process, depending onthe metallurgy of the UBM material. The resulting structure is a firstbump structure 20A including the first UBM layer 12A, the firstconductive pillar 16A, and the first solder layer 18A in accordance withan embodiment. After the bump formation, the first chip 100 may bebonded to another chip through chip-to-wafer level stacking orchip-to-chip level stacking or the like. It should be noted, however,that embodiments may be used in many different situations. For example,embodiments may be used in a chip-to-chip bonding configuration, achip-to-wafer bonding configuration, chip-level packaging, or the like.

FIGS. 5-6 are cross-sectional views at various stages of manufacturing asecond bump structure on a second chip according to at least oneembodiment.

Referring to FIG. 5, a second chip 200 includes a second semiconductorsubstrate 10B, a second UBM layer 12B formed on the second semiconductorsubstrate 10B, a mask layer 14 patterned on the second UBM layer 12B, asecond conductive pillar 16B formed in the opening of the mask layer 14,and a second solder layer 18B formed on the top of the second conductivepillar 16B. Any suitable process and materials may be used to form thestructures in the second chip 200, and those may be similar to or thesame as the formation in the first chip 100. In at least one embodiment,processes and materials used to form the elements 10B, 12B, 14, 16B and18B of the second chip 200 may be similar to or the same as theformation of elements 10A, 12A, 14, 16A and 18A of the first chip 100.

FIG. 5 also shows the formation of a metallization layer 22 on the topof the second solder layer 18B. The metallization layer 22 may be formedof a material highly reactive with solder material. In an embodiment,the metallization layer 22 is a Cu layer which is intended tosubstantially include a layer comprising pure elemental copper, coppercontaining unavoidable impurities, and/or copper alloys containing minoramounts of elements such as tantalum, indium, tin, zinc, manganese,chromium, titanium, germanium, strontium, platinum, magnesium, aluminumor zirconium. The formation methods may include sputtering, printing,electro plating, electroless plating, and/or commonly used depositionmethods. In one embodiment, the metallization layer 22 has a thicknessless than about 10 μm. In another embodiment, the thickness ofmetallization layer 22 is less than or equal to about 5 μm.

Next, as shown in FIG. 6, the mask layer 14 is removed, and then theuncovered portion of the second UBM layer 12B is etched. The resultingstructure is a second bump structure 20B including the second UBM layer12B, the second conductive pillar 16B, the second solder layer 18B andthe metallization layer 22 in accordance with an embodiment. After thebump formation, the second chip 200 may be bonded to the first chip 100through chip-to-wafer level stacking or chip-to-chip level stacking orthe like.

FIGS. 7-10 are cross-sectional views at various stages of bonding thefirst chip and the second chip by attaching the first bump structure tothe second bump structure according to various embodiments.

As shown in FIG. 7, in an embodiment of utilizing flip-chip bonding, thefirst chip 100 is flipped down toward the second chip 200 such that thefirst bump structure 20A faces the second bump structure 20B. Then abonding process is performed to bond the first chip 100 and the secondchip 200 by attaching the first bump structure 20A to the second bumpstructure 20B, and therefore a 3D chip stack 300 is formed throughjointing the first solder layer 18A and the metallization layer 22 asshown in FIG. 8. In an embodiment, the bonding process uses a solidliquid interdiffusion (SLID) technology. In an example of Cu/Sn binarymetal system, SLID technology is based on the rapid intermetallicformation between a high-melting point component (for example, Cu in themetallization layer 22) and a low melting point component (for example,Sn in the first solder layer 18A) at the temperature above the meltingpoint of Sn. As heat and pressure are applied, Sn is melted but Cu stillremains in solid phase. Solid liquid inter-diffusion occurs between thetwo phases, resulting in a phase transformation of the liquid componentto an intermetallic compound (IMC), which is strong enough to serve as abond and withstand elevated temperatures. When the temperature is raisedto the melting point of Sn, the interdiffusion process is acceleratedand the thickness of IMC increases rapidly. As depicted in FIG. 9,solidification occurs upon diffusion and reaction, and a firstintermetallic phase 24 (for example, Cu₆Sn₅) is initially developed inthe solder layers 18A and 18B at the processing temperature of 250˜300°C. As the interdiffusion continues, a second intermetallic phase 26 (forexample, Cu₃Sn) starts and finally becomes the fully IMC phase presentin a first IMC region 28A between the metallization layer 22 and thefirst conductive pillar 16A as shown in FIG. 10. The SLID process alsoforms a second IMC region 28B with the fully IMC phase between themetallization layer 22 and the second conductive pillar 16B because ofdiffusion and reaction of Cu and Sn in the second solder layer 18B. Inan embodiment, the fully formed IMC phase includes Cu and Sn havingreached a thermodynamic equilibrium and having long-term stability inaccordance with an embodiment. The first IMC region 28A has a thicknessgreater than or equal to about 0.5 μm. For example, the first IMC region28A has a thickness greater than or equal to about 1 μm. In alternativeembodiments of forming the conductive pillar 16A or 16B with the metalcapping layer 17 b, Sn tends to reacts with metals (for example, Ni) inthe metal capping layer 17 b during the thermal reflow process, and thusthe IMC phase in the regions 28A and/or 28B includes Cu, Sn and metal ofthe metal capping layer 17 b. For example, the IMC phase includes Cu, Snand Ni.

The bonding process results in the 3D chip stack 300 with a bondedinterconnection 32 that includes the first UBM layer 12A and the firstconductive pillar 16A of the first chip 100, the second UBM layer 12Band the second conductive pillar 16B of the second chip 200, and a jointstructure 30 between the first and the second conductive pillars 16A and16B. The joint structure 30 includes the first IMC region 28A, thesecond IMC region 28B and the metallization layer 22 between the IMCregions 28A and 28B. The metallization layer 22 can increase thesolid-liquid interdiffusion interfaces so as to fulfill the SLIDprocess, and therefore the solder migration phenomenon observed in hightemperature storage (HTS) test is eliminated and defect issues (such asopened bumps) are solved. The present embodiments of utilizing themetallization layer 22 can greatly improve the reliable performance ofthe bonded interconnection 32 in the 3D chip stack 300.

Some embodiments of the present disclosure relate to performing athermal reflow process on the first solder layer 18A before bonding thechips 100 and 200, and such methods of forming the 3D chip stack will bedescribed herein. FIGS. 11-13 are cross-sectional views at variousstages of bonding the first chip and the second chip by attaching thefirst bump structure with a hemisphere-shaped solder layer to the secondbump structure according to at least one embodiment, wherein likereference numerals refer to like elements.

With reference to FIG. 11, the formation of the first bump structure20A″ of the first chip 100 includes performing a thermal reflow processto melt the first solder layer 18A to form a hemisphere-shaped solderlayer 18A″, for example, either through heating or rapid thermalprocessing (RTP). During the thermal reflow process, Sn in the firstsolder layer 18A tends to react with Cu and or Ni in the firstconductive pillar 16A to form an IMC phase therebetween. In anembodiment of utilizing flip-chip bonding, the first chip 100 is flippeddown toward the second chip 200 such that the first bump structure 20A″faces the second bump structure 20B.

Then as shown in FIG. 12, a bonding process is performed to bond thefirst chip 100 and the second chip 200 by attaching the first bumpstructure 20A″ to the second bump structure 20B, and therefore a 3D chipstack 300″ is formed through joining the hemisphere-shaped solder layer18A″ and the metallization layer 22. In an embodiment, the bondingprocess uses a SLID technology with a Cu/Sn binary metal system. In someembodiments, intermetallic phases 24 and/or 26 are developed in thesolder layers 18A″ and 18B, and finally becomes the fully formed IMCphase present in a first IMC region 28A between the metallization layer22 and the first conductive pillar 16A, as shown in FIG. 13. The SLIDprocess also forms a second IMC region 28B with the fully formed IMCphase between the metallization layer 22 and the second conductivepillar 16B because of diffusion and reaction of Cu and Sn in the secondsolder layer 18B. In an embodiment, the fully formed IMC phase includesCu and Sn. In alternative embodiments, the fully formed IMC phaseincludes Cu, Sn and Ni. The bonding process results in the 3D chip stack300″ with a bonded interconnection 32 that includes the first UBM layer12A and the first conductive pillar 16A of the first chip 100, thesecond UBM layer 12B and the second conductive pillar 16B of the secondchip 200, and a joint structure 30 between the first conductive pillars16A and 16B. The joint structure 30 includes the first IMC region 28A,the second IMC region 28B and the metallization layer 22 between the IMCregions 28A and 28B.

According to some embodiments, a three-dimensional chip stack includes afirst chip having a first substrate and a second chip having a secondsubstrate. The first chip is bonded to the second chip to form a bondedinterconnection between the first substrate and the second substrate.The bonded interconnection includes a first conductive pillar overlyingthe first substrate, a second conductive pillar overlying the secondsubstrate, and a joint structure between the first conductive pillar andthe second conductive pillar. The joint structure includes a first IMCregion adjacent to the first conductive pillar, a second IMC regionadjacent to the second conductive pillar, and a metallization layerbetween the first IMC region and the second IMC region.

According to some embodiments, a method of forming a three-dimensionalchip stack, includes steps of: forming a first bump structure on a firstsemiconductor substrate, wherein the first bump structure includes afirst conductive pillar and a first solder layer on the top of the firstconductive pillar; forming a second bump structure on a secondsemiconductor substrate, wherein the second bump structure includes asecond conductive pillar, a second solder layer on the top of the secondconductive pillar, and a metallization layer on the second solder layer;attaching the first bump structure to the second bump structure; andperforming a thermal reflowing process to form a first IMC regionbetween the first conductive pillar and the metallization layer, and asecond IMC region between the second conductive pillar and themetallization layer.

According to some embodiments, a method of forming a three-dimensionalchip stack, includes steps of: receiving a first chip including a firstbump structure formed on a first semiconductor substrate, wherein thefirst bump structure includes a first conductive pillar and a firstsolder layer on the top of the first conductive pillar; receiving asecond chip including a second bump structure on a second semiconductorsubstrate, wherein the second bump structure includes a secondconductive pillar, a second solder layer on the top of the secondconductive pillar, and a metallization layer on the second solder layer;and bonding the first chip to the second chip by attaching the firstbump structure to the second bump structure. A first IMC region includescopper and tin formed between the first conductive pillar and themetallization layer, and a second IMC region includes copper and tinformed between the second conductive pillar and the metallization layer.

In accordance with a representative embodiment, a bonded structureincludes: a first substrate and a second substrate; a first conductivepillar over the first substrate, and a second conductive pillar over thesecond substrate, the first substrate coupled to the second substrate toform a bonded interconnection therebetween; and a metallization layerbetween the first conductive pillar and the second conductive pillar,the metallization layer having a first surface and a second surface, thebonded interconnection comprising a first joint structure between thefirst conductive pillar and the first surface of the metallizationlayer, the bonded interconnection further comprising a second jointstructure between the second conductive pillar and the second surface ofthe metallization layer, wherein material of the metallization layer hasa higher melting point than material of the first joint structure ormaterial of the second joint structure. The first joint structure maycomprise a first intermetallic compound (IMC), and the second jointstructure may comprise a second IMC. At least one of the first IMC orthe second IMC may comprise copper and tin. The metallization layercomprises a first material having a first melting point. The first IMCcomprises a second material having a second melting point, the secondmelting point lower than the first melting point. The second IMCcomprises a third material having a third melting point, the thirdmelting point lower than the first melting point. The first IMC maycomprise copper, tin, and nickel. The first substrate comprises a firstintegrated circuit device, and the second substrate comprises a secondintegrated circuit device. The bonded interconnection comprises thefirst integrated circuit device bonded to the second integrated circuitdevice. The metallization layer may comprise at least one of copper or alayer of elemental copper. At least one of the first conductive pillaror the second conductive pillar may comprise a copper pillar. The atleast one of the first conductive pillar or the second conductive pillarmay comprise a metal capping layer over the copper pillar. The metalcapping layer may comprise nickel.

In accordance with another representative embodiment, athree-dimensional chip stack includes: a first semiconductor substratehaving a first bump structure, the first bump structure comprising afirst conductive pillar and a first solder layer over the firstconductive pillar; a second semiconductor substrate having a second bumpstructure, the second bump structure comprising a second conductivepillar, a second solder layer over the second conductive pillar, and ametallization layer on the second solder layer, the metallization layerhaving a higher melting point than that of the first solder layer or thesecond solder layer; and the first bump structure coupled to the secondbump structure, wherein a first joint structure comprising a firstintermetallic compound (IMC) region comprising copper and tin is formedbetween the first conductive pillar and a first surface of themetallization layer, and a second joint structure comprising a secondIMC region comprising copper and tin is formed between the secondconductive pillar and a second surface of the metallization layer. Themetallization layer may comprise a copper layer. The first semiconductorsubstrate comprises a first integrated circuit device, and the secondsemiconductor substrate comprises a second integrated circuit device.The first bump structure and the second bump structure electricallycouples the first integrated circuit device to the second integratedcircuit device.

In accordance with yet another representative embodiment, a chip stackstructure includes: a first bump structure on a first semiconductorsubstrate, the first bump structure comprising a first conductive pillarand a first solder layer over the first conductive pillar; a second bumpstructure on a second semiconductor substrate, the second bump structurecomprising a second conductive pillar, a second solder layer over thesecond conductive pillar, and a metallization layer over the secondsolder layer; the first bump structure coupled to the second bumpstructure; and a first joint structure comprising a first intermetalliccompound (IMC) region between the first conductive pillar and a firstsurface of the metallization layer, and a second joint structurecomprising a second IMC region between the second conductive pillar anda second surface of the metallization layer, wherein the metallizationlayer has a higher melting point than the first IMC region or the secondIMC region. The metallization layer may comprise a copper layer, and thefirst IMC region may comprise copper and tin. The first conductivepillar may comprise a copper pillar. The first conductive pillar maycomprise a metal capping layer interposing the copper pillar and thefirst solder layer. The metal capping layer may comprise a nickel layer.

While the present disclosure has been particularly shown and describedwith reference to example embodiments thereof, a skilled person in theart will appreciate that there can be many embodiment variations of thisdisclosure. Although the embodiments and their features have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the embodiments.

The above method embodiments show representative steps, but they are notnecessarily required to be performed in the order shown. Steps may beadded, replaced, changed in order, and/or eliminated as appropriate, inaccordance with the spirit and scope of embodiments of the disclosure.Embodiments that combine different claims and/or different embodimentsare within the scope of the disclosure and will be apparent to thoseskilled in the art after reviewing this disclosure.

What is claimed is:
 1. A structure, comprising: a first substrate and asecond substrate; a first conductive pillar over the first substrate,and a second conductive pillar over the second substrate, the firstsubstrate coupled to the second substrate through the respectiveconductive pillars to form a bonded interconnection therebetween; and ametallization layer between the first conductive pillar and the secondconductive pillar, the metallization layer defining a first face and asecond face, the bonded interconnection comprising a first jointstructure between the first conductive pillar and the first face of themetallization layer, the bonded interconnection further comprising asecond joint structure between the second conductive pillar and thesecond face of the metallization layer, wherein material of themetallization layer has a higher melting point than material of thefirst joint structure or material of the second joint structure.
 2. Thestructure of claim 1, wherein the first joint structure comprises afirst intermetallic compound (IMC), and the second joint structurecomprises a second IMC.
 3. The structure of claim 2, wherein at leastone of the first IMC or the second IMC comprises copper and tin.
 4. Thestructure of claim 2, wherein: the metallization layer comprises a firstmaterial having a first melting point; the first IMC comprises a secondmaterial having a second melting point, the second melting point lowerthan the first melting point; and the second IMC comprises a thirdmaterial having a third melting point, the third melting point lowerthan the first melting point.
 5. The structure of claim 4, wherein thefirst IMC comprises copper, tin, and nickel.
 6. The structure of claim1, wherein the first substrate comprises a first integrated circuitdevice, and the second substrate comprises a second integrated circuitdevice.
 7. The structure of claim 6, wherein the bonded interconnectioncomprises the first integrated circuit device bonded to the secondintegrated circuit device.
 8. The structure of claim 1, wherein themetallization layer comprises at least one of copper or a layer ofelemental copper.
 9. The structure of claim 1, wherein at least one ofthe first conductive pillar or the second conductive pillar comprises acopper pillar.
 10. The structure of claim 9, wherein the at least one ofthe first conductive pillar or the second conductive pillar comprises ametal capping layer over the copper pillar.
 11. The structure of claim10, wherein the metal capping layer comprises nickel.
 12. Athree-dimensional chip stack, comprising: a first semiconductorsubstrate having a first bump structure, the first bump structurecomprising a first conductive pillar and a first solder layer over thefirst conductive pillar; and a second semiconductor substrate having asecond bump structure, the second bump structure comprising a secondconductive pillar, a second solder layer over the second conductivepillar, and a metallization layer on the second solder layer, whereinthe metallization layer having a higher melting point than that of thefirst solder layer or the second solder layer, wherein the first and thesecond semiconductor substrates are stackingly coupled through a firstjoint structure formed from the first solder layer and the metallizationlayer and a second joint structure formed from the second solder layerand the metallization layer, wherein the first and the second jointstructures respectively comprise a first intermetallic compound (IMC)region and a second IMC region comprising copper and tin.
 13. Thethree-dimensional chip stack of claim 12, wherein the metallizationlayer comprises a copper layer.
 14. The three-dimensional chip stack ofclaim 12, wherein the first semiconductor substrate comprises a firstintegrated circuit device, and the second semiconductor substratecomprises a second integrated circuit device.
 15. The three-dimensionalchip stack of claim 14, wherein the first bump structure and the secondbump structure electrically couples the first integrated circuit deviceto the second integrated circuit device.
 16. A chip stack structure,comprising: a first bump structure on a first semiconductor substrate,the first bump structure comprising a first conductive pillar and afirst solder layer over the first conductive pillar; a second bumpstructure on a second semiconductor substrate, the second bump structurecomprising a second conductive pillar, a second solder layer over thesecond conductive pillar, and a metallization layer over the secondsolder layer, the first bump structure and the second bump structurebeing configured to couple each other; and a first joint structurecomprising a first intermetallic compound (IMC) region between the firstconductive pillar and a first face of the metallization layer, and asecond joint structure comprising a second IMC region between the secondconductive pillar and a second face of the metallization layer, whereinthe metallization layer has a higher melting point than the first IMCregion or the second IMC region.
 17. The chip stack structure of claim16, wherein the metallization layer comprises a copper layer, and thefirst IMC region comprises copper and tin.
 18. The chip stack structureof claim 16, wherein the first conductive pillar comprises a copperpillar.
 19. The chip stack structure of claim 18, wherein the firstconductive pillar comprises a metal capping layer interposing the copperpillar and the first solder layer.
 20. The chip stack structure of claim19, wherein the metal capping layer comprises a nickel layer.